The art is replete with examples of Fischer-Tropsch catalytic processes for making hydrocarbons from H.sub.2 /CO mixtures. These processes yield olefinic aromatic and paraffinic hydrocarbons as well as oxygenated hydrocarbons. Much of this art concerns itself with methods for improving conversion (i.e., the amount of carbon monoxide converted to the above-mentioned products), or methods for improved selectivity (i.e., the amount of carbon converted to a given desired product divided by the total carbon converted).
Many examples in the art relate to improvements in the selectivity to a desirable product such as ethylene or materials which can be thermally cracked to ethylene. Materials which can be thermally cracked to ethylene include C.sub.2 -C.sub.4 paraffinic hydrocarbons and C.sub.3 -C.sub.4 olefinic hydrocarbons.
It is desirable to maximize the yield of ethylene in this process subgroup because losses occur when thermally cracking the other hydrocarbons to ethylene. An example of a process for maximizing the C.sub.2 -C.sub.4 olefins is U.S. Pat. No. 4,199,522, which is incorporated herein by reference. This patent discloses a method for increasing the selectivity to C.sub.2 -C.sub.4 olefins by using a catalyst with less than 100 m.sup.2 /g surface area and comprising at least one member of the group of metals, oxides or sulfides of molybdenum, tungsten, rhenium, ruthenium, nickel, palladium, rhodium, osmium, iridium, and platinum, and at least one member of the group of hydroxides, oxides or salts of alkali and alkaline earth metals; which catalyst may optionally be on a support comprising alumina, carbon, silica, zirconia, zircon, titanium dioxide, magnesia, or mixtures thereof.
An alternative to the above method is to selectively poison a conventional Fischer-Tropsch catalyst. The Fischer-Tropsch synthesis combines many types of reactions. Exemplary reactions include the formation and destruction of carbon-to-oxygen bonds, the formation and destruction of olefinic bonds, the formation of carbon-to-carbon bonds to yield polymers of methylene groups. An option to improve the yield of C.sub.2 -C.sub.4 olefins is to selectively poison the catalyst being used to prevent higher polymerization of the methylene groups.
U.S. Pat. Nos. 2,717,259 and 2,717,260, which are hereby incorporated by reference, describe two alternatives for doing this. The former discloses the addition of halogen to the Fischer-Tropsch reactants. The latter discloses the addition of halogen and sulfur to the Fischer-Tropsch reactants in combination. Both improvements increase the selectivity to C.sub.2 -C.sub.4 olefins. However, the improved selectivity has its price.
The halogen does not react irreversibly with the catalyst. The halogen in the form of hydrogen halide is mobile. The hydrogen halide may migrate from the catalyst into the product stream and corrode process equipment. The addition of sulfur to Fischer-Tropsch catalyst may substantially decrease the conversion. While the selectivity to C.sub.2 -C.sub.4 olefins may be higher, the net yield may be lower because of lowered conversion. If one attempts to restore conversion by raising temperature, the catalyst may degrade by coking and swelling. If one attempts to restore conversion by lowering the space velocity, the C.sub.2 -C.sub.4 olefin production still decreases.